Development of a silkworm silk fiber-reinforced poly(lactic acid) biocomposite

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Development of a silkworm silk fiber-reinforced poly(lactic acid) biocomposite


Author: Cheung, Hoi-yan Karen
Title: Development of a silkworm silk fiber-reinforced poly(lactic acid) biocomposite
Degree: Ph.D.
Year: 2009
Subject: Hong Kong Polytechnic University -- Dissertations.
Polymeric composites.
Biodegradable plastics.
Department: Dept. of Mechanical Engineering
Pages: xxx, 202, [18] leaves : ill. ; 30 cm.
Language: English
InnoPac Record:
Abstract: The development of biomaterials begins with those bioinert materials such as metals, ceramics and silicon etc. which are implanted permanently inside the host body without generating any adverse response and interaction with surrounding tissues. Nevertheless, the demand of biocompatible, biodegradable and bioresorbable materials has increased drastically since the last decade. A wide variety of natural biodegradable polymers like collagen, gelatin, chitosan and hyaluronic acid etc. and synthetic biodegradable polymers like poly(lactic acid) (PLA), poly(glycolic acid) (PGA) and their copolymers etc. have been investigated recently for medical and pharmaceutical applications. Synthetic biodegradable polymers have a promising advantage over natural polymers for the implant development because of their favorable properties, including good biocompatibility, biodegradability, bioresorbability, and mechanical and proliferation properties, are more predictable and reproducible. Their physical and chemical properties can be easily modified to achieve desirable mechanical and degradation characteristics. Additionally, synthetic polymers are generally degraded by simple hydrolysis, which is desirable as their degradation rate do not have variations from host to host, unless there are inflammations and implant degradation to affect local pH variations. They possess low or even negligible toxicity of products during in vivo degradation. PLA, a kind of well-recognized bio-degradable polymer, can maintain its mechanical properties even under a humid environment without being suffered for rapid hydrolysis. Therefore, PLA can be a potential structural material. PLA is a highly versatile biopolymer derived from renewable resources like starch or sugar-based materials such as corn. This biodegradable polymer is alpha polyester that is widely used in medical applications. During biodegradation, PLA degrades into lactic acid which is finally metabolized and excreted from the host as carbon dioxide and water. Natural/bio-fibers are classified into plant-based and animal-based fibers. Silkworm silk fiber (hereafter called 'silk fiber') is one of the animal-based natural fibers which is a potential candidate for structural composites and can be used for various medical applications such as wound sutures and biomedical scaffolds. In the aspect of modifying the thermal and mechanical properties of biodegradable polymers, silk-based fiber reinforced polymeric composites have emerged recently. As referred to those literatures, epoxy and natural or synthetic biodegradable polymers, such as alginate and poly(butylene succinate) (PBS) were mainly used as matrix for the composites. For PLA, several studies have reported that traditional synthetic fibers, like glass and recycled newspaper fibers, and plant-based natural fibers, like abaca and bamboo fibers, were used as reinforcements to enhance its mechanical properties. However, only few researches have attempted to use animal-based natural fibers as reinforcements for this biodegradable polymer. Mechanical properties of different types of cocoon spun silk fiber (both domesticated and wild) were investigated by performing a uniaxial tensile test on a single fiber. Scanning electron microscopy (SEM) was used to observe the morphology of two different types of silk fiber, and to measure their apparent diameters from which the cross-sectional area of the silk fiber for stress-strain analysis can be determined. The cross-sections of silk fibers can be assumed as circular, and wild silk fiber has a relatively high extensibility as compared with domesticated silk fiber and other natural fibers. Weibull analysis was also used to quantify tensile strength reproducibility of the silk fiber. Both single and twisted domesticated silk fibers have a better reproducibility of tensile properties than that of the wild silk fiber. In the current study, a thermosetting polymer, epoxy, was reinforced by chopped silkworm silk fibers to fabricate a non-biodegradable polymeric composite. Composites with different silk fiber contents were made by hand lay-up technique, and their mechanical properties were compared with that of pure epoxy sample. It was found that the use of silk fiber in a composite system can effectively enhance its mechanical properties and structural performance. The controls of fiber length and orientation as well as its surface modification may influence the mechanical properties of the composite. Besides, the control of resin's viscosity by the application of heat in a proper manufacturing process is essential to ensure high resin permeability into silk filaments. In addition, PLA was reinforced by chopped silk fibers to form a completely biodegradable and biocompatible biocomposite for tissue engineering applications. Mechanical and thermal properties of a silk fiber/PLA biocomposite were studied through different experimental approaches. Optimal values, in terms of fiber length and weight content of 5 mm and 5 wt. %, respectively to achieve the maximum micro-hardness of the biocomposite were obtained. The tensile property test revealed that the modulus of elasticity and ductility of the biocomposite substantially increased as compared with pure PLA. Moreover, the glass transition temperature of the biocomposite increased approximately by 10%. SEM images also showed that good interfacial bonding property between the silk fibers and PLA matrix was achieved, which reflected that a good wettability of the resin during extrusion and injection molding process occurred. A biodegradation test on silk/PLA biocomposites was performed. Physical and mechanical properties, pH condition of surrounding fluid and the morphology of fractured samples were studied at specific time intervals. The biodegradation rate of implants can be altered and their mechanical properties can be enhanced by incorporation of silk fiber. This is a potential solution to match with the degradation rate of PLA to the regeneration rate of neo-tissues. Theoretical predictions in determining the mechanical performance of silkworm silk fiber reinforced PLA biocomposites were conducted in order to validate the experimental results. Last but not the least, concluding remarks and suggestions for further study in the design and manufacturing of a biocomposite, biocompatibility tests in vitro and in vivo for cell attachment and proliferation for clinical applications are presented.

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